In recent years, a so-called lithium ion secondary battery using as a negative electrode a carbon material such as graphite and using as a positive electrode a lithium-containing metal oxide such as LiCoO2, which has a large capacity and is expected as a promising electrical storage device, has been used practically as the main power source for notebook computers and mobile phones. The lithium ion secondary battery is a so-called rocking chair battery such that after it is assembled, lithium ions are supplied from the lithium-containing metal oxide as the positive electrode to the negative electrode by charging the battery, and the lithium ions in the negative electrode are returned to the positive electrode by discharging the battery, and is characterized by having a high voltage, a large capacity and high safety.
Further, as concern for environmental problems is increasing, electrical storage devices (main power source and auxiliary power source) for electronic automobiles and hybrid electronic automobiles which replace gasoline-powered vehicles, have been actively developed, and lead batteries have been used as an electrical storage device for automobiles. However, due to enhancement of on-vehicle electric apparatus and devices, a new electrical storage device has been required in view of the energy density and the output density.
As such a new electrical storage device, attention has been paid to the above lithium ion secondary battery and an electric double layer capacitor. However, although a lithium ion secondary battery has a high energy density, it still has problems in output characteristics, safety and cycle life. On the other hand, an electric double layer capacitor has been used as a memory backup power source for IC or LSI, but the discharge capacity per charge is small as compared with a battery. However, it is maintenance-free and has high output characteristics which the lithium ion secondary battery does not have, such as excellent instantaneous charge and discharge characteristics and durability against charge and discharge for several tens of thousands cycles or more.
Although an electric double layer capacitor has such advantages, an energy density of a conventional electric double layer capacitor usually ranges from about 3 to about 4 Wh/l, which is lower than by two orders than that of a lithium ion secondary battery. In the case of an electrical storage device for electronic automobiles, an energy density of from 6 to 10 Wh/l is required for practical use and an energy density of 20 Wh/l for wide spread use.
As an electrical storage device to be used for such an application which requires a high energy density and high output-characteristics, attention has been paid to an electrical storage device also called a hybrid capacitor comprising a combination of storage principles of a lithium ion secondary battery and an electric double layer capacitor in recent years. A hybrid capacitor usually employs a polarized electrode for the positive electrode and a non-polarized electrode for the negative electrode, and attracts attention as an electrical storage device having both high energy density of a battery and high output characteristics of an electric double layer capacitor. Further, a hybrid capacitor has been proposed (Patent Documents 1 to 4) in which a negative electrode capable of absorbing and desorbing lithium ions is brought into contact with lithium metal so that lithium ions are preliminarily made to be absorbed and supported (hereinafter sometimes referred to as doping) by the negative electrode by a chemical or electrochemical method to lower the negative electrode potential, thereby to increase the withstand voltage and to significantly increase the energy density.
Such a hybrid capacitor is expected to shoe high performance, but has drawbacks such that when the negative electrode is doped with lithium ions, the doping requires a very long time, and it tends to be difficult to uniformly dope the entire negative electrode. Particularly, the doping is practically impossible to be carried out a large-size large capacity cell such as a cylindrical apparatus having electrodes wound or a rectangular battery having a plurality of electrodes laminated.
However, this problem has been dissolved all at once by an invention (Patent Document 5) such that the entire negative electrodes in the cell can be doped with lithium ions only by disposing lithium metal at the end of the cell, by forming pores penetrating from the front surface to the back surface on each of a negative electrode current collector and a positive electrode current collector so that lithium ions can move via the through pores, and further, by short circuiting the lithium metal as a lithium ion supply source and the negative electrode. Usually doping with lithium ions is carried out on the negative electrode, but Patent Document 5 discloses to conduct similarly the doping on the positive electrode together with the negative electrode or instead of the negative electrode.
Thus, there appears the prospect of the realization of such a capacitor that the entire negative electrodes in an apparatus can be uniformly doped with lithium ions in a short time even in a large-size cell such as a cylindrical apparatus having electrodes wound or a rectangular battery having a plurality of electrodes laminated, whereby the energy density with an improved withstand voltage will be drastically increased, and a large capacity will be achieved in combination with a high output density which an electric double layer capacitor inherently has.
However, in order to realize such a large capacity capacitor, a larger capacity, a higher energy density and a higher output density, and securement of higher durability are required.
Patent Document 1: JP-A-8-107048
Patent Document 2: JP-A-9-55342
Patent Document 3: JP-A-9-232190
Patent Document 4: JP-A-11-297578
Patent Document 5: WO98/033227